Low Carbon Aluminium Killed (LCAK), hot rolled steel sheets are commonly known and are used for the manufacture of a wide range of products such as steel pipes, tubes and automotive stampings etc. Many processes have been developed for making such steel sheets. These processes have focused primarily on increasing the yield strength of the resulting steel so as to impart high strength to the final product.
Examples of such processes are provided in U.S. Pat. Nos. 4,938,266 and 5,948,183, which are incorporated herein by reference. In each of these references, a process for making hot rolled steel sheets is provided. However, each of the processes is designed to provide steels with high strength. These references teach the use of various additives to assist the subject process. For example, Boron (B) is added to improve the hardenability of the steel since it prevents the excessive growth of crystal grains and prevents the precipitation of coarse carbides at high temperatures. Titanium (Ti) is another known additive that has been found to increase steel strength by precipitating dissolved Carbon to form Titanium carbide. However, for both B and Ti, a concentration exists below which the strength of the steel is reduced. With the advent of hydroforming processes, there has been a demand for high quality steel tubes that are more formable, i.e. having inter alia lower yield strength. Similarly, as automotive stampings become more complex, demand for higher formability (i.e. lower yield strength and higher elongation) steel has increased. In manufacturing low yield strength steel, it has been found that reducing free nitrogen is a contributing factor. One method of preparing such steel involves the addition of an element that precipitates the free nitrogen as a nitride. Examples of such additives are Aluminium, Titanium, Zirconium and Boron.
A major problem associated with the use of Boron is that the additions necessary to increase the formability of steel, also result in the formation of cracks in the cast slabs at a level significantly higher than typical with Boron free steel. These cracks develop into iron oxide defects also known as “slivers” in the final steel coil. Modifications to the casting process do not eliminate these defects. This results in a lower quality of steel. To remove the slivers, it is common to “scarf” the slabs (i.e. remove surface layer of steel) or to “slit” the resulting steel strip; i.e. reduce the width. In either case, a substantial yield loss is incurred and the processing time for the steel is increased.
Boron also results in increased rolling loads, which may cause hot-rolling problems such as crimps and folds that may limit the width that can be rolled in the hot mill.
The use of Boron and Titanium in steel has been known for many years but such use has been in a difference context.
As mentioned above, Boron is a very strong strengthener of steel. It has been used in ultra low Carbon steels, low Carbon steel and medium Carbon steel to give high strength. In order to achieve the strengthening effect, all free nitrogen must be removed. For this reason, sufficient or excess Titanium is added to combine with the nitrogen in the steel. This leaves the added Boron free to harden the steel. Although it is possible to harden steel by using less Titanium and more Boron, slab cracking results. Thus, for hardening steel, excess Titanium is used. A minimum amount of Boron is required to obtain the desired hardening effect, and this depends on the Carbon content.
The other application of Boron in a Titanium bearing steel is as an element used to control secondary work embrittlement in cold-rolled annealed interstitial-free (IF) steel. It is not added to lower yield strength in these steels. Titanium and/or niobium are added in sufficient quantities to remove all the nitrogen (N), all the Carbon (C) and all the sulphur (S) in de-gassed steel that has a very low N, C and S. However, the absence of interstitial elements such as Carbon makes the steel susceptible to cracking at grain boundaries during room temperature stamping. The addition of a few parts per million of Boron significantly decreases the temperature of transition from ductile fracture to brittle fracture. The level of Boron used for this application is far below the ranges used for softening LCAK (Low Carbon Aluminium Killed) steel These steels are also cold-rolled and annealed after hot rolling.
Titanium has strong infinity for oxygen. Thus, it can be used to remove oxygen from liquid steel in the same way that Aluminium is used. U.S. Pat. No. 4,001,052 for formable Boron-bearing steel teaches that Titanium, Zirconium or Aluminium could be used “kill” steel; i.e. remove oxygen from the molten steel. Boron was added to soften the steel. From a practical standpoint, Zirconium or Titanium would not be used to kill steel because the large quantities required would make either one prohibitively expensive. This patent expressed the Boron and Titanium contents as simple ranges and will result in some chemistries highly susceptible to cracking, others will have high rolling loads and others reduced formability as compared to non-Boron/Titanium alloyed steel.
Various other elements have been added to molten steel in addition to Boron and Titanium to improve the mechanical properties of steel. U.S. Pat. No. 6,007,644 teaches the manufacture of a high toughness and yield strength steel having a minimum yield strength of 325 Mpa (equivalent to 47.14 ksi). The yield strength is achieved by adding Vanadium (V) in addition to Titanium (Ti) to the molten steel. The Titanium is added to produce fine TiN precipitates which serve as nucleation sites for vanadium nitride, both of which are added to refine the austenite grain size which results in increased yield strength. However, given the range of nitrogen in the steel and the range of Titanium specified, the steel produced will result in inconsistent strength and frequent slivers when Boron is also present in this steel.
Another application of Boron in a Titanium bearing steel, as described in U.S. Pat. No. 4,375,376 is as an element for retarded aging in a cold rolled high yield strength steel product. The Boron is added most conveniently as solid particles of ferro-Boron. Titanium and Boron have also been added in the presence of phosphorous to produce deep drawing and high strength steel sheets by continuous annealing (Takahashi et al.).
Thus, while the above processes have focused primarily on increasing the yield strength of the resulting steel, there still exists a need for an improved method for making hot rolled steel having increased formability with a defect level not significantly different from non-Boron alloy steel.